MP4 is a final delivery lossy video format which is not suitable for editing in non-destructive video editing software like Final Cut Pro. Though Final Cut Pro 7 and Final Cut Pro X update to import MP4 video, it only support MP4 encoded with H.264. If the MP4 is encoded with other video codec, Final Cut Pro X/7 will still fail to import MP4.

Why convert them to other lossy formats if you can just convert the files to huffyuv or similar lossless codecs? Considering DV and Apple ProRes are already pretty huge. The difference to huffy isn't that big.